# -*- coding:utf-8 -*-
# Taken from Python 2.7 with permission from/by the original author.
import sys

from django.utils import six

def _resolve_name(name, package, level):
    """Return the absolute name of the module to be imported."""
    if not hasattr(package, 'rindex'):
        raise ValueError("'package' not set to a string")
    dot = len(package)
    for x in range(level, 1, -1):
        try:
            dot = package.rindex('.', 0, dot)
        except ValueError:
            raise ValueError("attempted relative import beyond top-level "
                              "package")
    return "%s.%s" % (package[:dot], name)


# 导入一个模块
if six.PY3:                                             # 在python3.x中，可以直接利用内置模块接口完成设置
    from importlib import import_module
else:
    # name的值为：django.db.backends.mysql
    def import_module(name, package=None):              # python2.x中，利用自定义完成接口import_module完成
        """Import a module.

        The 'package' argument is required when performing a relative import. It
        specifies the package to use as the anchor point from which to resolve the
        relative import to an absolute import.
        导入一个模块，
        参数'package'是需要的当采用一个相对导入时，
        """
        if name.startswith('.'):
            if not package:
                raise TypeError("relative imports require the 'package' argument")
            level = 0
            for character in name:
                if character != '.':
                    break
                level += 1
            name = _resolve_name(name[level:], package, level)
        __import__(name)
        return sys.modules[name]

# 基础知识补充：
# __import__():
# sys.modules[]:
